2018ApJ...862..147M

We use Atacama Large Millimeter Array CO(3-2) observations in conjunction with optical observations from the Hubble Space Telescope to determine the ratio of stellar to gas mass for regions in the Antennae Galaxies. We adopt the term "instantaneous mass ratio" IMR(t)=M_stars/(M_gas+M_stars), which is equivalent to the star formation efficiency (SFE) for an idealized system at t = 0. We use two complementary approaches to determining the IMR(t) based on (1) the enclosed stellar and molecular mass within circular apertures centered on optically identified clusters, and (2) a tessellation algorithm that defines regions based on CO emission. We find that only a small number of clusters appear to have IMR(0) = SFE > 0.2, which suggests that only a small fraction of these clusters will remain bound. The results suggest that by ages of 10^6.7 years, some clusters will have lost all of their associated molecular gas, and by 10^7.5 years this is true for the majority of clusters. There appears to be a slight dependence of the IMR(t) on the CO surface brightness, which could support the idea that dense molecular environments are more likely to form bound clusters. However, the IMR(t) appears to have a strong dependence on extinction, which likely traces the evolutionary state of clusters.